Generally, this is based on whether it was open when you last closed Word.
You could try my free Navigation Pane settings Add-In. It should give you control.
Navigation Pane Settings at Word Startup
Otherwise, you could add the following macro to your Normal.dotm:
Code:
Sub AutoExec()
Application.CommandBars("Navigation").Visible = False
End Sub
If that one does not work because of your Word settings, change the name from AutoExec to AutoNew.
Note, I've tested my Add-In and it works. It has been used by many since 2013. I have not tested the macro above but am writing them here off the top of my head. I think they will work.
Instructions for Installing Macros from Forums or Websites by Graham Mayor, MVP